I have a particular PHP page that I want to conditionally do things only if the visitor is not a search engine. Are there some good regex to match $_SERVER['HTTP_USER_AGENT']?

Or would it be better to do a javascript redirect back to the page but set a flag, since search engines don't have javascript? (I don't have to worry much about my users not having Javascript for this page.)

I don't want to completely exclude search engines from hitting the page though, so save the robots.txt suggestion.